Hi Romain - the challenge with cyclones is that the particle size split is a complex function of the pressure drop and the cyclone geometry, with the pressure drop being somewhat dependent on cyclone geometry.  By increasing the O/F discharge height, you will increase the pressure drop in the cyclone by 2-3 psig (depending on the COF density).  This may force more material to the cyclone apex, i.e. fluids will always flow to the point of least resistance, and change your cut size.  You'll also need to check that you don't overload the apex, a condition known as roping, which in turn forces excess material back up to the overflow.  My preference is always to have both O/F and U/F discharge to atmosphere.  However, for the sake of 1.5 m, and to avoid the need for an additional pump, perhaps you can make an exception, although you'll need to make sure you understand the impact on cut size, assuming cut size is the critical parameter for operating your cyclone.  You need to look at the entire pumpbox, pump, piping & cyclone configuration as a single system.  For example, a higher pressure drop means a higher tip speed on your pump to maintain the flow, which in turn will increase your wear rates, etc., etc.  I hope you find these comments useful.

Best Regards

Andrew 

Also see https://www.911metallurgist.com/grinding/introduction-2/ as it is somewhat related.

Romain, all Andrew wrote is correct. Furthermore, a taller cyclone overflow pipe will force your grind finer.

Thank you for your reply Andrew.

So I understand that with a outlet (at the atmosphère) +1m (instead of 1.5m in fact) from the hydrocyclone head, I will increase the pressure drop from 0.8 bars (design) to 1 bar (0.8bar + 3psi) and then increase the d50.

So if my pump is designed to supply 0.8 bar at the inlet of hydrocyclone, I won't have a good performance of hydrocyclone because my ideal pressure at the inlet is 1bar. Then a bad separation. 

I will check if I can just change the speed of the pump or the pulley-Vbelt. Or if I reduce my feed pipe diameter I should increase my pressure. Right ? 

Best Regards

Romain - I would adjust the pump speed to give you your target operating pressure, then sample around the cyclone and quantify how it is performing.  Depending on your downstream requirements, increasing the pump speed may be enough.  Obviously as the pump speed is increased, the volume being pumped increases (particularly if you are on the flat part of the pump curve), so you need to consider adding water to the pump box to maintain the level and avoid surging of the pump.  You don't need to consider things like reducing the feed pipe diameter.  Bottom line, make one change at a time, quantify the impact, and then decide what you are going to change next.  It's a complex system, although a relatively robust system, so if you proceed with caution you should be OK.  Best Regards - Andrew 

Ok, thank you for your help Andrew. 

Thank you for the links Jean, interesting. In fact it is not a grinding circuit but a classification line.